Non-intrusive stress measurement systems (NSMSs) are generally known for collecting structural data pertaining to gas turbine engine components. For instance, NSMS may be used to quantify any presence of blade vibration or deflection during operation, which may further be correlated to a quantified level of blade stress. Such analyses may be used for the purposes of engine and airfoil design, engine certification processes and/or engine health monitoring.
Blade vibration or deflection is typically derived in part from blade tip timing measurements as captured through an array of sensors, such as optical probes, or the like. For instance, the sensors of a typical NSMS are fixedly disposed within the engine case of an engine and used to detect the passing of the rotor blades or any other indicators externally provided on a rotor of an engine as it rotates during operation. Electrical signals that are output by the sensors can be used to determine the time or distance of arrival of each rotor blade, and the distance of arrival may further be used to determine blade deflection and stress.
As such, the accuracy of any blade stress analyses is dependent upon the integrity of the manner in which blade tip timing measurements are obtained. The integrity of blade tip timing measurements, however, may be compromised by any axial displacement of the blades during operation. Specifically, as engine speed increases, the axial position of the blades may axially move forward relative to the engine case, which may cause the NSMS sensor to interact with a different section of the rotor blade tip. As rotor blades are typically angled relative to its axis of rotation, an unknown offset in the axial position of the rotor may cause errors in the time or distance of arrival of the rotor blades, which may further compound errors in blade stress analyses.